The invention discloses a battery replacing system for a chassis of a power-driven passenger car and a battery replacing method. The system comprises a battery replacing platform and a battery replacing robot device; a battery replacing area is arranged on the battery replacing platform in the X-axis direction and is of a hollow structure, and conveying mechanisms are arranged on the two sides of the battery replacing area; the ends of the battery replacing area are each provided with a car wheel fixing mechanism, the heights of the car wheel fixing mechanisms are adjustable, and car wheels are fixed in the X-axis direction and the Z-axis direction; a rotating mechanism is arranged on the inner side of each car wheel fixing mechanism, and the car moves in the X-axis and Y-axis directions through the rotating mechanisms; and a car detecting device is arranged on the battery replacing area in the X-axis direction, and the battery replacing robot device is arranged at the lower end of the battery replacing platform to be used for replacing a battery of the power-driven passenger car in the battery replacing area. After accurate positioning in the X-axis, Y-axis and Z-axis directions is carried out, the battery is rapidly and accurately demounted and mounted by the battery replacing robot device at the fixed position through the lifting motion of the battery chassis, and the safe, accurate and rapid battery replacing method is provided for the power-driven passenger car.